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When I worked for the Ministry of Natural Resources back in the day, one of my summer jobs was to record the ages of pickerel/walleye via scales. Didn't take long to get squinty-eyed doing that! Tho I did get to use a microscope to do it.

For scale-less fish, they use the cleithrum - a bone behind the gills. The lines on it indicate age.

Well, it works for any fish, but scaling can be done on a live fish, whereas the cliethrum is used for dead specimens.
If you don't want to kill the fish, you have to use the less accurate weight/length to age charts for your area. You can usually find them online.

Last edited by sheamurai; 02-28-2014 at 04:55 PM.
Reason: more info added

Cool. I didn't know that tho I wonder how they would age a scaleless fish? Teeth?

There is an earbone called an Otolith that also grows as age rings however, each fish as their own bone structure so its going to vary fish to fish, and to view it you kind of have a dead fish when you are done. Some fish you can also view some growth rings on their bony structures like fins and spines, but again you pretty much end up killing the fish to find out.